| Work Detail |
Cadmium telluride solar cells typically rely on non-ohmic rear contacts that affect the open-circuit voltage of the device. Researchers in the UK attempted to solve this problem by adding a rear buffer layer of nickel oxide, which formed an ohmic rear contact and increased open-circuit voltage levels.
Researchers at Loughborough University in the UK have designed a cadmium telluride (CdTe) solar cell with a nickel oxide (NiO) buffer layer deposited without oxygen, which is said to significantly improve the open-circuit voltage of the device.
They explained that CdTe cells typically exhibit low open-circuit voltage levels because these devices rely on low minority carrier lifetimes, low carrier density, and non-ohmic back contacts. To solve this problem, they used NiO, which is an effective electron reflector due to the large conduction band shift, as a rear buffer layer to form an ohmic back contact.
“Optical transmission and band gaps are affected by oxygen ingress, typically decreasing in both respects with increasing oxygen,” the scientists explained. “NiO starts to show structural changes upon oxygen ingress, forming nickel vacancies. These characteristic changes suggest that there is a trade-off point that should be studied in relation to its use as a subsequent buffer layer for CdTe solar cells.”
The research group used the solar cell capacitance software SCAPS-1D, developed by Ghent University, to simulate a novel CdTe cell configuration. The 0.25 cm2 device was based on a glass and fluorine-doped tin oxide (FTO) substrate, a tin oxide (SnO2) electron transport layer (ETL), a CdTe absorber, the 100 nm thick Nio buffer layer and gold (Au) metal contacts.
In the proposed cell architecture, the NiO back buffer layer increases the device efficiency by reducing the barrier height at the Au back contact and enhancing the valence band shift at the CdTe/NiO interface.
“The NiO layer also gives rise to a large conduction band shift at the CdTe/NiO interface, thus forming an effective electron reflector to reduce interface recombination and increase the open circuit voltage,” the team explains, noting that the conduction band shift between NiO and CdTe is large.
“Between 0-20% oxygen, the value lies between 2.65 eV and 2.86 eV and between 40-60% it drops to 2.18 eV and 2.21 eV,” the academics further explained. “It increases again to 80% up to 2.62 eV. The large shift of the conduction band would directly enhance the open circuit voltage by being efficient electron reflectors, which prevents electron accumulation at the interface and reduces interface recombination.”
The new solar cell concept was presented in the study “ The effect of oxygen on NiO as a back buffer layer in CdTe solar cells,” published in the Royal Society of Chemistry . Looking ahead, the team plans to further improve the NiO layer by optimizing the substrate temperature during sputtering. |
